Processes for the manufacture of sodium or potassium metabisulfite are illustrated by U.S. Pat. Nos. 3,361,524, 3,860,695, 3,369,866 and 3,995,015.
U.S. Pat. No. 3,361,524 covers the simultaneous manufacture of sodium metabisulfite and sodium sulfite in a process having three reaction stages. In the first stage, sulfur dioxide and caustic soda are reacted to form sodium metabisulfite. The second stage reacts caustic soda with mother liquor and sulfur dioxide from the first stage to precipitate sodium sulfite. In the third stage, more caustic soda is added to crystallize more sodium sulfite.
U.S. Pat. No. 3,860,695 relates to the manufacture of sodium metabisulfite from sodium hydroxide or carbonate and sulfur dioxide. This process uses two stages of gas absorption. The first stage of sulfur dioxide absorption is carried out in a "jet scrubber". The gas is then forced to pass beneath the liquid level in the main body of the same vessel. This step constitutes a second stage of gas absorption. The pH is maintained in the range of 4 to 5. The sulfur dioxide content of the exit gas is 3.7% by volume, and thus further processing is required.
In U.S. Pat. No. 3,369,866, potassium hydroxide or carbonate and sulfur dioxide are reacted in potassium bisulfite solution at a pH of 4 to 7.5, and a temperature of 50.degree. to 80.degree. C. The solution is transferred to a second vessel where its pH is reduced to 5.5 by the addition of more sulfur dioxide in order to prevent precipitation of potassium sulfite, and the solution is cooled to 15.degree. to 25.degree. C. to obtain a crop of potassium metabisulfite crystals. The main concerns are to avoid product oxidation and the inclusion of sulfite in the product. No mention is made of crystal size.
The process of U.S. Pat. No. 3,995,015 is a considerable simplification and improvement over the prior art. Only two processing vessels are used, and the formation as an intermediate of heavy slurries of sodium sulfite is advantageously avoided. The process uses a sulfur dioxide-containing gas and soda ash as starting materials. Most sulfur dioxide absorption and product crystallization occur simultaneously in a reactor. In a scrubbing vessel, the 10 to 20% of the feed sulfur dioxide stream not absorbed in the reactor is absorbed in a slurry of said ash in mother liquor. However, the sodium metabisulfite product typically contains 20 to 30% minus U.S. Standard 100 mesh screen, has the characteristic odor of sulfur dioxide, and is irritating to those physically handling it. As a result, dust masks, respirators and fairly elaborate ventilation systems should be used.
Therefore, there is a continuing need for an improved process for the manufacture of sodium metabisulfite. In particular, there is a need for a process that produces a coarser product. Such a product should be less irritating to those handling it. Thus, the discovery of such an improved process would constitute a significant contribution to the art.